Roll for treatment of paper webs or the like

ABSTRACT

A roll for pressure treatment of paper webs has a stationary shaft which is spacedly surrounded by the concave cylindrical internal surface of a rotary sleeve. The shaft carries a row of pressure shoes which define therewith plenum chambers for a hydraulic fluid serving to urge the convex cylindrical external surfaces of the shoes toward the internal surface of the sleeve. Each external surface has a centrally located primary hydrostatic bearing pocket which communicates with the respective plenum chamber and a set of four auxiliary hydrostatic bearing pockets which surround the respective primary pocket and receive only that hydraulic fluid which overflows the lands between the primary pocket and the auxiliary pockets. The pockets in each external surface are surrounded by a marginal portion of such surface and the primary pocket has arms which extend to the marginal portion and each of which is flanked by a pair of auxiliary pockets. The arms of the primary pocket are parallel to the longitudinal axis of the sleeve and/or to an axis which is normal to the longitudinal axis. The auxiliary pockets form groups which make oblique angles with the two axes. If the external surfaces of the shoes are rectangular, the groups of auxiliary pockets extend substantially diagonally of the respective external surfaces.

CROSS-REFERENCE TO RELATED CASE

The roll of the present invention constitutes an improvement over and afurther development of the roll which is disclosed in the commonly ownedU.S. Pat. No. 4,064,607 granted Dec. 27, 1977 to Robert Wolf for"Compression roller for paper producing machinery".

BACKGROUND OF THE INVENTION

The present invention relates to rolls for the treatment of runningwebs, especially to improvements in rolls for pressure treatment ofrunning paper webs in paper making and/or processing machines.

U.S. Pat. No. 3,846,883 granted Nov. 12, 1974 to Biondetti discloses aroller for pressure treatment of webs. The roller comprises a flexibleshaft and a rigid sleeve which spacedly surrounds the shaft and rotatesin response to engagement with a running web of paper or the like. Themeans for transmitting forces between the sleeve and the shaft comprisesseveral bearing shoes which have convex cylindrical surfaces adjacent tothe concave cylindrical internal surface of the sleeve. In theembodiment which is illustrated in FIG. 5 of the patent to Biondetti,the convex cylindrical surface of each shoe has several hydrostaticbearing pockets which are filled with pressurized hydraulic fluid toprevent metal-to-metal contact between the sleeve and the shoes. Rollsof the just outlined character can be used in paper making machines toexpel surplus moisture from freshly formed paper webs or in calenders toenhance the smoothness of or to otherwise treat the surfaces of runningwebs consisting of paper or the like. Furthermore, such rolls can beused in many other types of machines wherein discrete sheets orcontinuous webs or strips of flexible material are caused to passthrough the nips of rolls which are driven to advance the webs or sheetsor which receive motion in response to entrainment of sheets or webstherebetween.

The purpose of bearing shoes between the shaft and the rotatable sleeveor shell is that the sleeve is much less likely to undergo deformationso that the width of the nip remains constant from one end and all theway to the other end of the roll. Furthermore, the bearing shoes renderit possible to vary the pressure against the internal surface of thesleeve from section to section, as considered in the axial direction ofthe roll, so as to ensure that the resistance to deformation is greatestin the region where the sleeve is most likely to be deformed. This alsocontributes to the possibility of ensuring that the width of the nipremains constant all the way between the ends of the cooperating rolls.

The external cylindrical surface of each bearing shoe is complementaryto the internal cylindrical surface of the shell. In the embodimentwhich is shown in FIG. 5 of the patent to Biondetti, the externalsurface of each bearing shoe has a centrally located hydrostatic bearingpocket and two or more additional bearing pockets in circumferentialorientation around the central pocket. The patentee suggests that thissubstantially reduces friction between the stabilizing edge around thecentral pocket and the shell. The two additional pockets which areactually shown in FIG. 5 of Biondetti are disposed at the opposite sidesof the central pocket, as considered in the circumferential direction ofthe shell.

FIG. 3 of U.S. Pat. No. 3,802,044 granted Apr. 9, 1974 to Spillmann etal. discloses a modified bearing shoe with a circular external surfacewhich is formed with four equal sector-shaped hydrostatic bearingpockets. Two of the pockets are halved by a plane which includes thelongitudinal axis of the sleeve, and the remaining two pockets arehalved by a plane which is normal to the longitudinal axis of thesleeve. The webs or lands which separate the neighboring pockets fromeach other make angles of approximately 45 degrees with the longitudinalaxis of the sleeve as well as with a transverse axis which is normal toand intersects the longitudinal axis. It can be said that the lands orwebs between the neighboring sector-shaped pockets extend diagonally ofthe exposed cylindrical surface of the bearing shoe.

The hydrostatic pockets which are disclosed by Biondetti and Spillmannet al. can stabilize the respective bearing shoes. Such shoes must becapable of performing various movements relative to the shaft. As arule, the bearing shoes are mounted for movement radially of the shaftas well as for tilting movement not unlike the parts of universaljoints. The stabilizing effect of the hydrostatic pockets is to be feltin several directions, namely, as considered in the axial direction aswell as in the circumferential direction of the sleeve (i.e., in thedirection of as well as at right angles to the central longitudinal axisof the sleeve). Otherwise stated, the hydrostatic bearing pockets of theaforediscussed conventional bearing shoes ensure that the axis of eachbearing shoe extends exactly radially of the associated sleeve or shell.The patent to Spillmann et al. further discloses the possibility ofrotating the array of pockets shown in FIG. 3 through 45 degrees so thattwo of the lands between neighboring bearing pockets would be parallelto and the remaining two lands would extend at right angles to thelongitudinal axis of the sleeve. It is suggested that such orientationof the lands is more likely to prevent undesirable tilting in a planewhich is normal to the longitudinal axis of the sleeve.

An object which is common to the inventions disclosed in theaforediscussed patents is to ensure that the marginal portion of theexposed external surface of each bearing shoe be maintained at aconstant distance from the internal surface of the sleeve, i.e., thateach and every portion of such marginal portion be disposed at apredetermined distance from the sleeve. This is intended to guaranteethe formation of a liquid film having a constant thickness and toprevent direct surface-to-surface (metal-to-metal) contact between thebearing shoes and the sleeve. It has been found that the proposals ofBiondetti and Spillmann et al. cannot ensure total absence ofmetal-to-metal contact between the bearing shoes and the sleeve underany and all circumstances which arise when a roll of the above outlinedcharcter is in actual use. In many instances, the shoes are likely todestroy the liquid films between their external surfaces and theinternal surface of the sleeve, especially when the roll is idle (i.e.,when the sleeve does not rotate about the shaft) and/or during theinitial stage of acceleration of the sleeve from zero speed.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to provide a novel and improved roll whichcan be used for the treatment of running paper webs or the like andwherein direct contact between the bearing shoes and the rotary sleeveis prevented or its likelihood reduced even under those circumstanceswhich normally entail such contact in conventional rolls.

Another object of the invention is to provide a pressure applying rollwherein the bearing shoes are constructed, mounted and configurated insuch a way that they are held out of direct contact with the sleeve evenif the roll is idle and/or when the sleeve of the roll begins to rotate.

A further object of the invention is to provide a novel and improvedbearing shoe for use in a roll of the above outlined character.

An additional object of the invention is to provide a machine,especially a paper processing machine, which embodies a roll with one ormore bearing shoes of the above outlined character.

Another object of the invention is to provide a novel and improveddistribution of hydrostatic bearing pockets on the surfaces of bearingshoes in the above outlined rolls.

A further object of the invention is to provide a roll whose useful lifeis longer than the useful lives of heretofore known rolls.

Another object of the invention is to provide a novel and improvedsystem for supplying pressurized hydraulic fluid to the hydrostaticbearing pockets in the bearing shoes of rolls of the above outlinedcharacter.

An ancillary object of the invention is to provide a novel and improvedpressure roll which can be installed in many types of existing papertreating, paper making and/or other machines as a superior substitutefor presently employed rolls.

The invention is embodied in a roll for pressure treatment of webs,especially paper webs. The roll comprises an elongated shaft which canbe mounted in a stationary frame or in movable bearings of a calender orthe like, and a hollow cylindrical sleeve having a longitudinal axis, atransverse axis normal to the longitudinal axis, and a concavecylindrical internal surface spacedly surrounding the shaft. The rollfurther comprises at least one bearing shoe which is interposed betweenthe shaft and the sleeve and is movable substantially radially of aswell as tiltable relative to the shaft. The shaft and the shoe define aplenum chamber which contains a supply of pressurized hydraulic fluidserving to urge the shoe toward the internal surface of the sleeve. Inaccordance with a feature of the invention, the shoe has a convexcylindrical external surface which is adjacent to the internal surfaceof the sleeve and is formed with a substantially centrally locatedprimary hydrostatic bearing pocket communicating with the plenum chamberso that it is filled with pressurized fluid which maintains the externalsurface of the shoe out of direct metal-to-metal contact with theinternal surface of the sleeve, and at least three but preferably moreauxiliary hydrostatic bearing pockets surrounding the primary pocket andreceiving only such fluid which leaks from the primary pocket by flowingalong lands which form part of the external surface of the shoe andseparate the primary pocket from the auxiliary pockets. The primarypocket has arms which are flanked by pairs of auxiliary pockets, andsuch arms extend in parallelism with the one and/or other axis of thesleeve. The auxiliary pockets form groups of two which extend at obliqueangles to the axes of the sleeve. The external surface of the shoe has amarginal portion which surrounds the pockets, and the arms of theprimary pocket extend all the way to the marginal portion of theexternal surface.

The shoe is preferably formed with a single channel which connects theplenum chamber with the central portion of the primary pocket.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its mode ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic transverse sectional view of a roll which embodiesone form of the present invention;

FIG. 2 is a fragmentary axial sectional view of the roll, substantiallyas seen in the direction of arrows from the line II--II of FIG. 1;

FIG. 3 is a greatly enlarged plan view of a bearing shoe in the roll ofFIGS. 1 and 2, substantially as seen in the direction of arrow III inFIG. 1;

FIG. 4 is a similar plan view of a modified bearing shoe;

FIG. 5 is a plan view of a third bearing shoe; and

FIG. 6 is a similar plan view of a fourth bearing shoe.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 and 2, there is shown a portion of a rollwhich can be used in a machine for the making and/or processing of paperwebs or the like. For example, the roll can be utilized with severaladditional rolls to form part of a calender wherein the neighboringrolls define one or more nips through which the web- or strip-shapedmaterial passes during treatment. The roll of FIGS. 1 and 2 comprises ahollow cylindrical sleeve or shell 1 which spacedly surrounds astationary shaft or carrier rail 2. The end portions of the shaft 2 aremounted in a suitable frame and the end portions of the shell 1 aremounted on suitable bearings which surround the adjacent portion of theshaft 2. Alternatively, and as disclosed in German patent applicationSer. No. P 30 07 112.6, the stationary frame can be replaced byshiftable bearings for the end portions of the shaft 2. Such bearingsare utilized in calenders.

The means for transmitting forces between the concave cylindricalinternal surface 1a of the shell 1 and the shaft 2 comprises a batteryor row of discrete bearing elements or shoes 3 of which only one isshown in the drawing. Reference may be had to the aforementioned U.S.Pat. No. 3,802,044 to Spillmann et al. wherein FIG. 1 shows a shaft theend portions of which are mounted in a stationary frame and which issurrounded by a hollow cylindrical sleeve. Bearings are interposedbetween the end portions of the sleeve and the corresponding portions ofthe shaft. FIG. 1 of this patent further shows a row of five bearingshoes within the confines of the cylindrical sleeve. For the sake ofcompleteness, the disclosure of this patent is incorporated herein byreference.

The exposed upper side or surface 3a of the bearing shoe 3 which isshown in FIGS. 1 and 2 constitutes a portion of a convex cylindricalsurface which is complementary to the concave cylindrical internalsurface 1a of the shell 1. The surface 3a has a rectangular outline (seeFIG. 3) with two longer sides 3a' which are parallel to the axis X--Xand two shorter sides 3a" which are parallel to the axis Y--Y. The axisX--X is parallel to the longitudinal axis of the shell 1, and the axisY--Y is normal to the axis X--X. FIG. 3 shows that each of the axes X--Xand Y--Y divides the surface 3a into a pair of mirror symmetricalhalves.

The shoe 3 has a second surface 3b which is located opposite the surface3a and is adjacent to but spaced apart from the external surface of theshaft 2. The surface 3b has a blind bore 3c which receives a piston 4having a piston rod 4a rigid or integral with the shaft 2. Theperipheral surface of the piston 4 is rounded so that the shoe 3 can betilted with reference to the shaft 2. Furthermore, the shoe 3 can moveradially of the shaft 2, i.e., in the axial direction of the piston 4and its piston rod 4a. Such universal movability of the shoe 3 withreference to the piston 4 is desirable in order to compensate forflexure of the shaft 2 in response to transmission of substantialbending forces from the shell 1 via bearing shoe 3.

If desired, the surface 3b of the bearing shoe 3 can be provided with aradially inwardly extending projection constituting a plunger which isreceived, with requisite clearance, in a cylindrical bore machined intothe peripheral surface of the shaft 2.

The shaft 2 has an axial bore 10 which supplies pressurized hydraulicalfluid to a radial bore 10a extending through the shaft 2, through thepiston rod 4a and piston 4 and serving to deliver pressurized fluid to aplenum chamber 8 constituting the outermost portion of the bore 3c inthe surface 3b of the shoe 3. If the illustrated piston 4 is replaced bya plunger extending into a radial bore of the shaft 2, the plenumchamber constitutes the radially innermost portion of the bore in theshaft 2.

The purpose of hydraulic fluid in the plenum chamber 8 is to bias theshoe 3 toward the internal surface 1a of the sleeve 1. However, directmetal-to-metal contact between the shoe 3 and the sleeve 1 is preventedby the provision of several hydrostatic bearing pockets or recesseswhich are machined into the surface 3a of the shoe 3 and are illustratedin FIG. 3. These pockets include a centrally located primary pocket 6and four auxiliary or satellite pockets 7 surrounding the pocket 6. Thecentrally located primary pocket 6 has a cruciform shape with a centralportion surrounding a radially extending channel or port 5 whichconnects the pocket 6 with the plenum chamber 8. In addition, the pocket6 has four extensions or arms 9 which extend between the neighboringpairs of auxiliary pockets 7. It will be noted that the surface 3a has aframe-like marginal portion 12 which surrounds the auxiliary pockets 7and the arms 9 of the central pocket 6 and is adjacent to the sides 3a'and 3a" of the surface 3a. In addition, the surface 3a has foursubstantially L-shaped surface portions or lands 11 each of whichseparates an auxiliary pocket 7 from two neighboring arms 9 of theprimary pocket 6. As can be seen in FIG. 3, the auxiliary pockets 7receive only such hydraulic fluid which overflows the respective lands11, i.e., only such fluid which escapes or leaks from the central pocket6.

The auxiliary pockets 7 can be said to form two pairs or groups ofpockets which extend diagonally of the surface 3a and make obliqueangles with the axes X--X and Y--Y. This can be readily seen in the planview of FIG. 3. The tip or outermost portion of each arm 9 extends allthe way to the respective part of marginal portion 12 of the surface 3a.It will be noted that those parts of the marginal portion 12 which areadjacent to the sides 3a' are somewhat narrower than the parts ofmarginal portion 12 which are adjacent to the shorter sides 3a".However, this is not crucial, i.e., the width of the shorter parts ofthe marginal portion 12 can equal or be less than the width of thelonger parts. The auxiliary pockets 7 are disposed at the four cornersof the surface 3a, and the primary pocket 6 substantially fills thespace within the confines of the four auxiliary pockets 7. The featurethat the auxiliary pockets 7 are disposed at the corners of the surface3a and are, therefore, remote or reasonably remote from the axes X--Xand Y--Y is desirable and advantageous because such distribution ofauxiliary pockets contributes to the restoring action of the hydrostaticpockets upon the orientation of the shoe 3 relative to the sleeve 1.Such distribution of the auxiliary pockets 7 and such configuration ofthe centrally located primary pocket 6 contributes to the stabilizingeffect of the hydrostatic bearing pockets upon the orientation of theshoe 3 relative to the sleeve 1. The auxiliary pockets 7 have arectangular outline because the surface 3a is rectangular. However, itis equally possible to increase the width of the shorter arms 9 and toutilize auxiliary pockets which have a substantially square outline. Itis further possible to utilize a centrally located pocket havingrelatively narrow as well as relatively wide arms, i.e., the width ofall four arms 9 need not be the same.

The feature that the arms 9 are parallel with the axes X--X and Y--Y isdesirable and advantageous because the fluid which fills such arms canexert a very pronounced and practically immediate restoring action uponthe orientation of the shoe 3 relative to the internal surface 1a of thecasing 1 as soon as the shoe 3 is only slightly inclined with referenceto the shell 1, i.e., as soon as the width of a portion of the gapbetween the surfaces 1a and 3a is not uniform throughout the entirewidth and length of the surface 3a.

An important advantage of the improved roll is that each bearing shoe 3must be provided only with a single port or channel 5 for admission ofpressurized fluid from the respective plenum chamber into the centralportion of the associated hydrostatic bearing pocket 6. Though FIG. 5 ofthe aforementioned U.S. patent to Biondetti shows what appears to be asimilar design (this Figure does not show discrete ports or channels foradmission of pressurized fluid into the additional bearing pockets), thepatent does not disclose any means for ensuring adequate delivery offluid to each additional pocket. It has been found that the provision ofrelatively long lands 11 between the primary pocket 6 and the auxiliarypockets 7 shown in FIGS. 1 to 3 ensures adequate supply of fluid in eachof the auxiliary pockets. This is attributable to the novelconfiguration of the primary pocket 6, i.e., this pocket has severalarms 9 which are flanked by pairs of auxiliary pockets 7 so that thelength of the lands 11 is considerable with attendant greater likelihoodof overflow of fluid from the pocket 6 into the pockets 7. In any event,the rate of flow of fluid from the pocket 6 into the pockets 7 along therespective lands 11 is at least as pronounced as or greater than therate of flow of fluid from the pockets 7 and the interior of the sleeve1 (i.e., along the marginal portion 12 of the surface 3a).

The absence of discrete connections between the plenum chamber 8 and theauxiliary pockets 7 is desirable and advantageous because this ensuresthat the pressure of fluid in the primary pocket 6 exceeds the pressureof fluid in the auxiliary pockets. Also, the pressure differentialbetween the primary pocket 6 and the plenum chamber 8 is preferablysmall, i.e., the cross-sectional area of the port or channel 5 isrelatively large so that the port 5 is not likely to be clogged when theroll is in use. Absence of clogging further reduces the danger ofmetal-to-metal contact between the head 3 and the sleeve 1.

The aforediscussed orientation of arms 9 (so that they extend in thelongitudinal direction of and at right angles to the axis X--X of thesleeve 1) also contributes to more satisfactory establishment andretention or maintenance of a fluid film between the surfaces 1a and 3a.Such orientation of the arms 9 renders it possible to assemble theauxiliary pockets 7 into groups or pairs of pockets which extenddiagonally of the surface 3a, i.e., these groups make oblique angleswith the axes X--X and Y--Y. It has been found that such orientation ordistribution of the arms 9 and of the groups of auxiliary pockets 7contributes to even more satisfactory stabilization of the bearing shoe3 relative to the sleeve 1, especially as regards the tendency of theshoe 3 to turn about the axis of the piston 4 (i.e., about an axis whichis normal to the axis X--X). Mere retention of a bearing head 3 in aposition in which the axis of the bearing head is normal to the axisX--X does not suffice to invariably prevent direct contact between thesurfaces 1a and 3a because holding of the shoe against movement fromsuch radial position does not exclude rotation of the shoe about its ownaxis (i.e., about the axis of the bore 3c) and the resultingmetal-to-metal contact with the sleeve.

The shoe 3 could be held against rotation about its axis by mechanicalmeans, e.g., by the provision of suitable abutments or stops which wouldhold the shoe against rotation about an axis which is normal to the axisX--X. However, it has been found that the solution which is shown inFIGS. 1 to 3 is simpler, more reliable and less expensive. British Pat.No. 1,359,839 discloses an annular hydrostatic bearing pocket with acluster of relatively small triangular additional pockets therearound.The additional pockets can be said to form groups or pairs which extenddiagonally of the exposed surface of the bearing shoe. However, theadditional pockets receive pressurized fluid by way of narrow channelswhich are equipped with flow restrictors so that the likelihood ofclogging of such narrow channels with impurities which are contained inthe hydraulic fluid is very pronounced. Moreover, the hydrostaticpockets of the British patent are provided in a concave surface formingpart of a bearing for the convex cylindrical external surface of aroller.

The bearing shoe 13 of FIG. 4 has a rectangular surface 13a which formspart of a convex cylindrical surface and has a centrally located primarypocket 16 with four extensions or arms 19 and four auxiliary orsatallite pockets 17 of triangular outline. The pockets 17 are locatedat the four corners of the surface 13a which has a frame-like marginalportion 22 and four lands or intermediate portions 21. Each land 21extends between a pair of extensions 9 and the respective marginalpocket 17. The primary pocket 16 resembles a diamond and the tips of itsarms 19 extend all the way to the respective parts of the marginalportion 22 of the surface 13a. The reference character 5 again denotes aport or channel which supplies pressurized hydraulic fluid from a plenumchamber (not shown) to the primary pocket 16. The auxiliary pockets 17receive only such fluid which overflows the respective lands 21.

FIG. 5 illustrates the surface 23a of a circular bearing shoe 23 whichcan be used as a substitute for the shoe 3 or 13. The primary pocket 26of the surface 23a receives pressurized hydraulic fluid by way of a portor channel 5 and has two extensions or arms 29 which extend in thedirection of the transverse axis Y--Y. The surface 23a is further formedwith four arcuate auxiliary pockets 27 which are separated from eachother and from the central pocket 26 by relatively narrow lands 25. Thereference character 32 denotes the marginal portion of the surface 23a.It will be noted that the tips of the arms 29 of the central pockets 26again extend to the respective parts of the marginal portion 32 of thesurface 23a.

It goes without saying that the round bearing shoe 23 of FIG. 5 can bereplaced with an oval shoe or with a shoe which resembles an oval orcircular body but has a relatively large number of facets.

FIG. 6 shows a fourth bearing shoe 33 which is similar to the shoe 23 ofFIG. 5 and has a cylindrical convex surface 33a formed with a cruciformprimary pocket 36 having four arms 29a and with four auxiliary pockets37 separated from the primary pocket by relatively narrow lands 36. Thereference character 32a denotes the marginal portion of the surface 33a.As explained in connection with FIG. 5, the shoe 33 can be replaced witha shoe having an oval or polygonal outline whereby the number of thesides or facets greatly or substantially exceeds the number of sides ofthe shoe 3 or 13. The arms 29a include a pair of arms which are parallelwith the axis X--X and two arms which are parallel with the axis Y--Y.The advantages of such distribution of arms 29a are the same as those ofthe arms 9 shown in FIG. 3.

In many instances, the auxiliary pockets receive requisite quantities ofhydraulic fluid if the primary pocket is formed with only two extensionsor arms. However, the primary pockets which are shown in FIGS. 3, 4 and6 are preferred at this time because they invariably ensure adequatefilling of auxiliary pockets with hydraulic fluid. As a rule, themachining of primary pockets of cruciform shape is simpler and lessexpensive than the machining of otherwise configurated primary pockets.

The utilization of bearing shoes whose external surfaces have apolygonal (particularly a rectangular) outline has been found to behighly advantageous for several reasons. First of all, suchconfiguration is relatively simple and, furthermore, the auxiliarypockets 7 or 17 can be located at a considerable distance from thecenter (port 5) of the respective primary pocket 6 or 16. Such placingof auxiliary pockets at a substantial distance from the axis of therespective shoe reduces the likelihood of rotation of the shoe about itsaxis, i.e., the fluid in the auxiliary pockets 7 or 17 is most likely toprevent turning of the shoe 3 or 13 about the axis of the port 5. Theutilization of bearing heads having rectangular external surfaces withauxiliary pockets at the four corners and a cruciform primary pocket hasbeen found to constitute one of the most satisfactory solutions ofpreventing undesirable turning of the shoe about its own axis, i.e.,about the axis of the respective liquid-supplying port. It will bereadily appreciated that the length of the lands 11 in FIG. 3 is greaterthan the length of the lands 21 on the convex surface 13a shown in FIG.4 because the primary pocket 16 of the shoe 13 does not have a cruciformshape. Moreover, resort to a cruciform primary pocket renders itpossible to employ relatively large auxiliary pockets which alsocontributes to the stablizing effect of the auxiliary pockets, i.e., toprevention of rotation of the shoe about its own axis. A comparison ofFIGS. 3 and 4 shows that the length of those lands 11 which separate twoarms 9 of the primary pocket 6 from a given auxiliary pocket 7 equals orapproximates the length of that part of the marginal portion 12 whichsurrounds the given auxiliary pocket. In other words, the likelihoodthat the rate of escape of fluid from an auxiliary pocket 7 along thecorresponding part of the marginal portion 12 would exceed the rate offlow of fluid from the primary pocket 6 into such auxiliary pocket ismuch less pronounced than in the embodiment of FIG. 4.

The effective cross-sectional area of a central or primary pocket ispreferably between 0.7 and 1.5 times the effective cross-sectional areaof the respective plenum chamber. This is desirable and advantageousbecause such ratio ensures the establishment of a satisfactory pressuredifferential between the interior of the primary pocket and the interiorof the respective plenum chamber.

We claim:
 1. A roll for pressure treatment of webs, especially paperwebs, comprising an elongated shaft; a hollow cylindrical sleeve havinga longitudinal axis, a transverse axis normal to said longitudinal axis,and a concave cylindrical internal surface spacedly surrounding saidshaft; and at least one bearing shoe interposed between said shaft andsaid sleeve, said shoe being movable substantially radially of andtiltable relative to said shaft and defining with said shaft a plenumchamber for a supply of pressurized hydraulic fluid which urges saidshoe in a direction toward said internal surface, said shoe having aconvex cylindrical external surface adjacent to said internal surfaceand including a substantially centrally located primary hydrostaticbearing pocket communicating with said plenum chamber and at least fourisolated auxiliary hydrostatic bearing spockets spaced so as toeffectively surround said primary pocket, said external surface furtherincluding a marginal portion surrounding said pockets and landsseparating said primary pocket from said auxiliary pockets, said primarypocket having arms extending to said marginal portion, flanked by saidauxiliary pockets and substantially parallel with at least one of saidaxes, said primary pocket further having a central portion and said shoehaving a single channel connecting said plenum chamber with the centralportion of said primary pocket, said auxiliary pockets being isolatedfrom the primary pocket without direct hydraulic fluid communicationtherebetween and being placed so as to receive fluid which leaks fromsaid primary pocket and overflows along the respective lands between theprimary pockets and the auxiliary pockets.
 2. The roll of claim 1,wherein said primary pocket has four arms each of which is flanked bytwo auxiliary pockets, two of said arms being substantially parallelwith one of said axes and the remaining two arms being substantiallyparallel with the other of said axes.
 3. The roll of claim 1, whereinsaid primary pocket has a cruciform shape.
 4. The roll of claim 1,wherein said external surface has a substantially rectangular outline.5. The roll of claim 4, wherein said external surface has two relativelylong sides substantially parallel to one of said axes and two relativelyshort sides substantially parallel with the other of said axes.
 6. Theroll of claim 1, wherein the effective cross-sectional area of saidprimary pocket is between 0.7 and 1.5 times the effectivecross-sectional area of said plenum chamber.
 7. The roll of claim 1,wherein said primary pocket has the shape of a diamond.
 8. The roll ofclaim 1, wherein said external surface has a substantially circularoutline.
 9. The roll of claim 8, wherein said auxiliary pockets arearcuate recesses in said external surface and said primary pocket has atleast two arms each of which is flanked by two auxiliary pockets.